Processes for the production of polymerizable materials often involve process equipment on which polymer tends to accumulate during normal process operation. This accumulation often interferes with the proper functioning of the process equipment by locking moving parts in static positions or restricting fluid flow required for proper functioning of the process equipment. Many of these processes involve one or more process fluids, including a liquid phase fluid, which may be used to advantage.
In particular, despite efforts to prevent formation of polymer during purification of polymerizable monomers in separation apparatus, such as a distillation column, the unwanted formation and accumulation of polymer is a common and troublesome occurrence. For example, one type of polymerization, “condensation polymerization” which forms “condensation polymer,” is especially troublesome when distilling polymerizable monomers. Monomers known to undergo condensation polymerization during distillation include, but are not limited to: (meth)acrylic acid and its esters, vinyl chloride, hydrogen cyanide, (meth)acrylonitrile, (methyl)styrene, and other vinyl monomers. Condensation polymerization occurs when vapor-phase monomers condense onto equipment surfaces, in the absence of suitable polymerization inhibitor compounds, and then undergo uninhibited polymerization, forming condensation polymer solids. Condensation polymer will continue to accumulate on the equipment surfaces as long as condensation in the process equipment continues unchecked.
A particularly vexing and dangerous problem is the accumulation of polymer on and near safety equipment, such as pressure relief devices. The purpose of pressure relief devices is to minimize damage to process equipment, such as distillation columns when abnormal pressure conditions occur, such as, for example, high pressure (over-pressure) or abnormally low pressure (vacuum) conditions. Pressure relief devices avoid explosion or implosion of process equipment under abnormal pressure conditions by opening a path of fluid communication between the interior and exterior of the process equipment (i.e., a valve will open, a rupture disk will rupture or burst open, a bursting panel will burst), thereby allowing equalization of internal pressure with the ambient pressure.
It is very important to prevent or minimize the accumulation of polymer on and near safety devices, such as pressure relief devices, to ensure their correct and safe operation. For example, the accumulation of polymer in and on mounting connections for pressure relief devices may result in an undesirable seal between the mounting connections and the pressure relief devices, impairing removal and replacement of the safety devices. Additionally, polymer may accumulate upstream and proximate to the safety device, such as on the inner walls of a conduit or recessed region wherein the safety device is positioned. Accumulation of polymer in either area tends to restrict the free flow of process fluid near and to the safety device and may, therefore, prevent the pressure relief from operating properly in response to process fluid pressure. This situation limits the capacity of the pressure relief device and creates an unsafe condition. Polymer may also adhere to the surfaces of the pressure relief devices themselves, preventing them from operating as intended—for example, polymer accumulation may cause relief valves to “stick” in the closed position or may cause rupture disks to fail to open at their intended burst pressure.
When distilling polymerizable monomers in distillation columns having pressure relief devices accumulation of polymer on and near pressure relief devices should be avoided or minimized to ensure their correct and safe operation, thereby also ensuring safe, long-term operation of distillation columns.
One known method for combating condensation polymerization in the vapor-containing regions of process equipment such as storage tanks, reaction kettles, and distillation columns, is to maintain the surface temperature of the equipment above the dew point of the monomer(s) being produced. Jacketing, insulation and electric or steam tracing of process equipment has been shown to be relatively effective in “open-flow” regions, i.e., regions where the vapor-phase monomer can freely flow away from the heated surfaces. This approach is not effective, however, in low flow regions such as proximate to pressure relief device mounting connections, where the monomer vapors may stagnate and become trapped. This situation is further exacerbated when the mounting connection is on the top head of a distillation column, such that the upstream surface of the pressure relief device is oriented horizontally.
Addition of vapor-phase polymerization inhibitors has also been utilized as a means for preventing condensation polymerization inside process equipment, such as distillation columns. Examples of such vapor-phase polymerization inhibitors include, but are not limited to, sulfur dioxide (SO2) used in hydrogen cyanide production processes, and one or more salts of N-nitrosophenylhydroxylamine (NPH) such as the ammonium salt, used in (meth)acrylic acid production processes. The efficacy of adding vapor-phase polymerization inhibitors, however, is limited because the fluid present in low flow regions, such as near nozzles, or in interior corners, is essentially stagnant and, therefore, poor mixing occurs and the inhibitors tend not to flow into these regions, whereby polymer formation and accumulation may proceed virtually unabated.
Another method for combating condensation polymerization in monomer distillation columns is using of internal spray devices to spray liquid on interior surfaces of process equipment, thereby washing away condensate before polymerization can occur. U.S. Pat. No. 6,409,886 B1 discloses a spraying and supplying means positioned inside a distillation column which sprays liquid on interior surfaces of the distillation column, with the intent of inhibiting polymer formation. By design, however, this type of spray device is most effective in minimizing polymer formation in large, open-flow regions, such as the top head of a distillation column or storage tank. This approach fails, however, to adequately prevent the accumulation of material in small, low-flow regions, such as the pressure relief device mounting connections on the top head of the column and on the opposite-facing surfaces of process apparatus. This is because the spray devices tend to be installed such that the direction of spray flow is downward, obliquely downward, or may be even sideways, but rarely upward and always linear. In such circumstances, surfaces above the spray devices are often not sprayed with liquid. Similarly, the surfaces of process equipment that do not at least partially face the spray device are not sprayed with liquid. As mentioned below, additional spray devices may be used, but this approach introduces other difficulties.
In those cases where accumulations must also be prevented on or near small process connections, such as vessel nozzles and valves, U.S. Pat. No. 6,409,886 B1 further suggests that one or more spray nozzles may be employed to specifically spray these connections. However, the installation of the spray nozzle is mechanically complex as well as invasive to the vessel, requiring penetration of the vessel at a separate place on the vessel wall, and the positioning of the line(s) inside the vessel for supplying the liquid to the spray nozzle. With this approach, the elevation and alignment of the spray nozzle itself, as well as the flow rate and spray pattern, are critical and, in practice, it is quite difficult to obtain the proper combination of these variables such that polymer accumulation is wholly prevented. While the effectiveness of such an arrangement could be improved through the use of multiple spray nozzles and an overwhelmingly large quantity of liquid spray, such an approach is costly and impractical in commercial operations. Another limitation of this approach is that it requires a continuous supply of flushing fluid to operate as intended. If there is a significant interruption in the supply of flushing fluid, the benefit of the disclosed apparatus and method may not be maintained and the service-readiness of the pressure relief device may be compromised.
Additionally, the physical presence of the spray nozzle(s) and its supply line(s) create an undesirable obstruction in the process connection, thereby interfering with the free flow of material through the attached pressure relief device, as well as providing additional surfaces on the components themselves for the accumulation of condensation polymer. In some instances, the direct impingement of pressurized spray liquid on the relief device may also cause mechanical fatigue, thereby shortening the service life of the device and leading to premature failures. While the apparatus and method disclosed in U.S. Pat. No. 6,409,886 resolve many problems of the prior art, because of the above-stated limitations, this apparatus and method are impractical when applied, for example, to the small, low flow regions that lie upstream of pressure relief devices in distillation columns.
Thus, there remains a long-felt need to provide a simplified, reliable, inexpensive and effective method for minimizing accumulation of polymer and condensation of polymerizable materials on process equipment, such as pressure relief devices, thereby ensuring the service-readiness of the pressure relief devices while in use in process apparatus during production of polymerizable monomers. It would be desirable to employ a more foolproof, passive approach to preventing the accumulation of polymer on and near pressure relief devices. The method of the present invention overcomes the deficiencies of the prior art, while meeting the need of the chemical process industries for a method of safely distilling polymerizable monomer.